1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing a vitreous silica crucible, and more specifically relates to a technique appropriate for inner surface property control in manufacturing a vitreous silica crucible for pulling a silicon single crystal.
2. Description of Related Art
In general, a silicon single crystal has been manufactured by the Czochralski method (the CZ method) using a vitreous silica crucible. In this method, polycrystalline silicon raw material is charged into a vitreous silica crucible, the raw material is melted to form silicon melt, a seed crystal is dipped into the silicon melt and the seed crystal is gradually pulled up, while rotating the seed crystal. The seed crystal acts as a crystal core to grow a silicon single crystal.
The vitreous silica crucible has a two-layer structure including an outer layer containing a number of bubbles and a transparent inner layer. A surface property of the inner layer contacting silicon melt in pulling a silicon single crystal largely affects a property of the resultant silicon single crystal and a yield of silicon wafers. Thus, conventionally, the inner layer is made of synthetic vitreous silica formed by amorphous synthetic silica powder, and the outer layer is made of natural vitreous silica.
Conventionally, in melting silicon and pulling a single crystal, there has frequently occurred a problem of melt surface vibration. According to this problem, dipping a seed crystal is disturbed by a wave generated on a liquid surface of silicon melt, and thus pulling of a silicon single crystal is prevented or single crystallization is deteriorated. This melt surface vibration (liquid surface vibration) phenomenon has become more likely to occur in accordance with increase in diameter of a silicon crystal, the demand to improve an inner surface property of a vitreous silica crucible has become stronger.
When a vitreous silica crucible is manufactured by the rotating mold method, silica powder is deposited inside the rotating mold to form a silica powder layer with a predetermined thickness, and thereafter the silica powder layer is melted by arc discharge, and cooled to be solidified. In the manufacturing process of a vitreous silica crucible, silica fumes are generated when a silica powder raw material is heated by arc discharge. So, in general, a manufacturing apparatus for a vitreous silica crucible includes an outlet tube to release gases and residues such as fumes from a furnace inside. However, the fumes tend to attach to electrodes or furnace walls within the furnace of the manufacturing apparatus, and the fumes can be peeled off and drop into the mold. When the fumes attached to a silica powder layer in the mold are melted and solidified, portions of the inner surface of the resulting vitreous silica crucible will have a deteriorated property.
Similar to the above-mentioned problem, when a silicon single crystal is pulled by the CZ method using a vitreous silica crucible with defects in the inner surface property, there has occurred a problem that dipping a seed crystal is disturbed by a wave generated on a liquid surface of silicon melt. Because of the problem, pulling of a silicon single crystal is prevented or single crystallization is deteriorated.
Nowadays, increase in diameter of a silicon single crystal has been demanded to produce wafers with a large diameter of 30 inches (76.2 cm) to 40 inches (116 cm). The increase in diameter requires a larger amount of powder in melting a silica powder layer, and requires a larger amount of powder to be applied to electrodes.
However, when a larger amount of powder is applied to electrodes, vibration generated in the electrodes increases. Thus, impurities attached to the electrodes become more likely to drop to the mold by the vibration. When the impurities are melted and solidified, the inner surface property of a vitreous silica crucible largely deteriorates. When a silicon single crystal is pulled using a vitreous silica crucible with defects in property, there occurs a problem in growth of a silicon single crystal.
JP-A-2002-154894 discloses a technique to remove silica fumes generated on an inner surface of a vitreous silica crucible to prevent the above-mentioned problems.
Furthermore, JP-B-2811290 and JP-B-2933404 disclose a technique to use amorphous synthetic silica powder to form an inner surface of a vitreous silica crucible to improve an inner surface property of a vitreous silica crucible.